The present invention relates to an electrode structure of a reverse conducting GTO (gate turn-off thyristor) device in which a GTO and a reverse conduction diode are integrally formed in the same semiconductor wafer.
A reverse conducting GTO device is a device in which a GTO and a diode for allowing a curret to flow in the direction opposite to a forward current of this GTO are integrally formed, namely, a GTO and a reverse conduction diode for reverse conduction of the GTO are integrally formed in the anti-parallel connection state. For example, such a GTO device is disclosed in Japanese Patent Disclosure (Kokai) No. 51-38985 and the like. An example of a typical arrangement of this reverse conducting GTO device is shown in FIG. 1.
A GTO section a has a four-layer structure consisting of a p.sup.+ -type first emitter layer 11, an n-type first base layer 12, a p-type second base layer 13, and an n.sup.+ -type second emitter layer 14. The second emitter layer 14 is divided into a plurality of parts. A reverse conduction diode section b is constituted by an anode layer 13' consisting of the p-type layer which is common to the second base layer 13 in the GTO section a, an n-type layer 12' which is common to the first base layer 12 in the GTO section a, and an n.sup.+ -type cathode layer 15. An anode electrode 18 also serves as an anode electrode of the GTO section a and a cathode electrode of the diode section b and is commonly provided for the first emitter layer 11 in the GTO section a and the n.sup.+ -type cathode layer 15 in the diode section b. A cathode electrode 16 is provided on each segment part of the second emitter layer 14. A gate electrode 17 is provided on the second base layer 13 in the GTO section a. An anode electrode 19 is provided on the anode layer 13' in the diode section b. The anode electrode 19 and the cathode electrode 16 are electrically connected and have the same potentials. An isolation region c is arranged between the GTO section a and the diode section b for prevention of mutual interference between the GTO section a and the diode section b. An n.sup.+ layer 20 is formed in the isolation region c to substantially separate the second base layer 13 and the anode layer 13'. Practically speaking, the n.sup.+ layer 20 is provided to prevent both the gate electrode 17 and cathode electrode 16 in the GTO section a, from being shortcircuited through the anode layer 13' in the diode section b when a negative bias is applied. There is also a case where a groove to substantially separate the second base layer 13 and the anode layer 13' is formed in the isolation region c in place of the n.sup.+ layer 20. The cathode electrode 16, gate electrode 17 and anode electrode 18 are respectively connected to a cathode terminal K, a gate terminal G and an anode terminal A for connection with the outside.
Such a reverse conducting GTO device is used as, for instance, a main switching element in an inverter for converting a direct current to an alternating current or the like. For example, in a GTO inverter in which an output is supplied to an inductive load such as an induction motor, a diode is usually connected in anti-parallel with the GTO. The reverse conducting GTO device is optimum as such a main switching element for an inverter. Inverters generate AC electric power in front of a square wave pulse train that is derived by switching the DC power source and are used as, e.g., a driving power source of an induction motor serving as a power source of an electric car. In such an inverter, to obtain a good AC waveform with fewer unnecessary harmonic components and less electric power loss, a number of complicated square wave pulses to be combined. Therefore, the switching of the main switching element of the inverter is fairly complicated and particularly in a multiphase inverter such as a three-phase inverter or the like for obtaining a multiphase alternating current, the switching of the main switching element becomes extremely complicated. In addition, in the multiphase inverter, in the case where an output is supplied to an inductive load such as a transformer to synthesize an output or an induction motor, the voltages and currents which are applied to the main switching element of the inverter and to the diode connected in anti-parallel therewith become very complicated.
In the case of using the reverse conducting GTO device in such an inverter, there is a case where a positive voltage is applied to the GTO section, for instance, after the forward current flowed through the diode section in the state whereby the GTO section is off. In such a case, even when the positive voltage is applied to the GTO section, the off state of the GTO section has to be maintained.
FIG. 2 shows a waveform of a voltage V.sub.A which is applied to the GTO section and a waveform of a current I.sub.D flowing through the diode in one example of such a case.
As shown by solid lines in the diagram, the forward current I.sub.D flows through the diode section while the GTO section is in the off state, and after time t.sub.1, the positive voltage is applied to the GTO section, so that the applied voltage V.sub.A of the GTO section is recovered. However, in case of the rate of decrease in the current I.sub.D of the diode section is large as indicated by broken lines in the diagram, the GTO section is erroneously ignited after time t.sub.1 and the blocking capability could not be maintained. This is because the excessive carriers in the diode section act as a trigger current of the GTO section.
Namely, during the period when the diode current I.sub.D is flowing, positive holes flow from the anode layer 13' in the diode section to the cathode layer 15, while electrons flow from the cathode layer 15 to the anode layer 13', respectively. As shown in FIG. 2, at time t.sub.1, the anode-cathode voltage of the GTO section becomes opposite to that before time t.sub.1 and the potential on the anode side becomes higher than that on the cathode side. At this time, the excessive electrons existing in the diode section b are drained from the cathode layer 15 in the diode section and the excessive holes are drained from the anode layer 13', respectively. However, the excessive carriers overflowed into the region near the n.sup.+ layer 20 formed in the isolation region c and into the GTO section a are not returned to the diode section b. In other words, the excessive electrons pass through the first emitter layer 11 and go out of the anode electrode 18, thereby allowing the positive holes, as many as these excessive electrons, to enter. On one hand, the excessive holes pass through the second base layer 13, gate electrode 17 near the isolation region c and a resistor R.sub.GK (not shown) connected between the gate and cathode, and then they are drained to the cathode electrode 16. In the ordinary case, the resistor R.sub.GK is connected between the gate and cathode on the outside of the element in order to improve the dv/dt withstanding capacity of the GTO section and to raise the forward withstanding voltage. Thus, the displacement current in association with the recovery of the voltage of the GTO section and the current due to the drain of the excessive holes are added and the resultant added current flows through the resistor R.sub.GK. When the voltage drop due to the current flowing through the R.sub.GK exceeds the minimum gate trigger voltage corresponding to the built-in potential of the junction consisting of the second base layer 13 and second emitter layer 14, the positive holes pass from the second base layer 13 through the second emitter layer 14 and enter the cathode electrode 16, thereby allowing the electrons, as many as these holes, to enter the second base layer 13 from the second emitter layer 14. The GTO is erroneously ignited due to the above-described operation. This misignition can easily occur as a decrease rate dI.sub.D /dt of the diode current I.sub.D becomes large. This is because as the decrease factor dI.sub.D /dt of the diode current I.sub.D becomes large, the residual quantity of the excessive carriers (remaining in the diode section b and in the isolation region c) increases and in particular, the residual quantity of the holes (whose mobility is smaller than that of the electrons) increases.
To avoid such a problem, the width of the isolation region c is generally made wide in order to prevent the excessive carriers in the diode section b from exerting an influence on the GTO section a. However, since the isolation region c completely serves as a dead space with respect to the operations and functions of the GTO section and diode section themselves, when the width of the isolation region c is set to be large, the substantial areas of the GTO section and diode section become small. Consequently, this causes problems such that a large enough current capacity cannot be assured and the on-state voltage becomes large and the like.